The man held responsible for the post-9/11 anthrax attacks may have had a secret accomplice, or been completely innocent of his alleged crimes, according to research into the FBI's investigation of the affair.

Bruce Ivins, an army bio-defence expert, committed suicide in 2008 after learning that murder charges were about to be filed against him in connection with the high-profile terrorist campaign, in which five people were killed and another 17 injured.

But an article published this week in the Journal of Bioterrorism & Biodefence highlights several inconsistencies in the forensic evidence against Dr Ivins, raising speculation that the FBI got the wrong man, and then prematurely closed their investigation following his death. The report, co-authored by three scientists, says that detectives failed to properly analyse the dried anthrax spores that were used in the attacks, which took place over several weeks following the September 11 bombings in 2001.

Analysis of the white powder, which was sent through the postal service to news organisations and politicians, showed that it contains unexpected traces of tin. That suggests a high degree of manufacturing skill, contrary to official conclusions that the attacks were part of a relatively-unsophisticated campaign carried out by Dr Ivins alone.

Agency scientists initially described the tin as an "element of interest" in the case, according to internal FBI documents uncovered by The New York Times. Early on in their investigation, they regarded it as a crucial clue which suggested that the anthrax – which had been mailed in envelopes containing the message: "Death to America ... Death to Israel... Allah is great" – had come from a relatively-professional source.

They later dropped that line of inquiry, however, and never mentioned the tin publicly. Following the death of Dr Ivins, who killed himself as the FBI were preparing to indict him, the agency failed to provide any detailed explanation of how the anthrax was manufactured.

The Journal's article will add to speculation that Dr Ivins was innocent of his alleged crime. An eccentric, with a history of erratic behaviour and some circumstantial links to the attacks, sceptics say he made a convenient scapegoat for investigators under pressure to close what became a long-running case.

Dr Ivins had an office near the New Jersey post box where two of the contaminated letters originated, and worked unusually late hours on the nights before they had been sent. He had also spent much of his career studying anthrax and had sometimes referred to a schizophrenic alter ego called "Crazy Bruce."

There was, however, no concrete evidence linking Dr Ivins to the crime. A report published last year by the National Academy of Sciences concluded that the FBI did not have enough scientific evidence to produce a conviction, had the case gone to trial.

Among their many criticisms, the authors found that the link between the anthrax used in the attacks and a supply which Dr Ivins kept in his lab was "not as strong" as the agency suggested.